Amorphous copolyesters comprising terephthalic acid (T) residues and diol residues comprising varying ratios of 1,4-cyclohexanedimethanol (CHDM) residues and ethylene glycol (EG) residues are well known in the plastics marketplace. As used herein, the abbreviation PETG refers to copolyesters comprising terephthalic acid residues as the diacid residue component and a diol residue component comprising up to 50 mole percent CHDM residues with the remainder EG residues. PCTG refers to copolyesters comprising T residues and a diol residue component comprising greater than 50 mole percent CHDM residues with the remainder being EG residues. Copolyesters comprising T residues and diol residues comprising about 20 to 70 mole percent CHDM residues and about 80 to 30 mole percent EG residues are amorphous. The term “amorphous” as defined herein means a polyester that does not exhibit a substantial crystalline melting point when scanned by differential scanning calorimetry (DSC) at a rate of 20° C./minute.
Amorphous copolyesters in general possess a combination of desirable properties for many applications. These properties include excellent clarity and color, toughness, ease of processing, and chemical resistance. Accordingly, amorphous copolyesters are known to be useful for the manufacture of extruded sheet, packaging materials, and parts for medical devices, etc. Application in transparent medical parts requires resistance to craze formation and mechanical failure when exposed to lipid and/or isopropyl alcohol (IPA) solutions. Whereas amorphous copolyesters are known in the art to have good resistance to these chemicals and are widely applied in these applications, craze formation occurs at high strains and is thus an area of needed improvement. Consequently, there is an unmet need for amorphous copolyesters that under high strains have improved resistance to lipid and IPA solutions.
There is also an important need for amorphous copolyesters that have improved resistance to hydrolytic degradation. U.S. Pat. No. 5,656,715 discloses that copolyesters containing a diol residue component comprising 60 to 100 mole percent residues of 1,4-cyclohexanedimethanol exhibit improved resistance to hydrolytic degradation.
Neopentyl glycol (NPG-2,2-dimethylpropane-1,3-diol) has been used in combination with EG and terephthalic acid to form amorphous copolyesters. However, the combination of NPG and CHDM as the diol component of copolyesters has received minimal attention. Several early references disclose copolyesters comprising both CHDM and NPG residues and terephthalic acid residues. Example 46 of U.S. Pat. No. 2,901,466 describes a copolyester of unknown composition that was reported to have a crystalline melting point of 289-297° C. U.S. Pat. No. 3,592,875 discloses copolyester compositions that contain both NPG and CHDM residues with an added polyol present for branching. U.S. Pat. No. 3,592,876 discloses polyester compositions that contain EG, CHDM and NPG residues with the NPG residue level limited to up to 10 mole percent. U.S. Pat. No. 4,471,108 discloses low molecular weight polyesters some of which contain CHDM and NPG residues, but also contain a multifunctional branching agent. U.S. Pat. No. 4,520,188 describes low molecular weight copolyesters comprising mixtures of aliphatic and aromatic acid residues with both NPG and CHDM residues present. Japanese Patent Publication JP 3225982 B2 discloses amorphous copolyesters which are said to be useful in the formulation of coating compositions for steel sheet. The disclosed copolyesters comprise a diacid component comprising mixtures of aliphatic and aromatic acid residues and a diol component comprising NPG and CHDM residues.
U.S. Pat. No. 4,551,403 discloses low molecular weight polyesters that are used as binders or as the matrix resin for photosensitive materials for electrophotography. Example 1 of this patent discloses the preparation of a polymer from dimethyl terephthalate, neopentyl glycol and 1,4-cyclohexanedimethanol. The polymer is said to have a reduced viscosity of 0.35 as measured at 30° C. in tetrachloroethane at a polymer concentration of 0.5%. The reduced viscosity of 0.35 corresponds to an inherent viscosity as defined herein of 0.29 dL/g using equations provided by F. W. Billmeyer, Textbook of Polymer Science, John Wiley & Sons, (1971) page 84. The low molecular weight polyesters disclosed by U.S. Pat. No. 4,551,403 are not useful for producing shaped articles by injection molding or extrusion procedures.
EP-A1-411,136 discloses copolyesters comprising diacid residues comprising mainly of terephthalic acid residues and glycol residues comprising 15 to 85 mole percent C-2 to C-16 aliphatic diol residues and 85 to 15 mole percent 1,4-cyclohexanedimethanol residues wherein 80 mole percent of the cyclohexanedimethanol residues are in the trans form. It is believed that the properties of the copolyesters specifically exemplified in EP-A1-411,136 are inferior to the copolyesters described herein. More specifically, the copolyesters described herein possess improved melt strength and resistance to hydrolysis as compared to the copolyesters of the examples of EP-A1-411,136.